Typical of this type of multiplex transmission system is a LAN (Local Area Network) transmission system. A typical LAN include CAN (Control Area Network) which is used for data transmission in a car.
In the multiplex transmission system based on the CAN, the transmission waveform for high speed applications (e.g., high-speed transmission at a transmission speed of 1 Mbps) is defined, and two multiplex transmission lines (buses) are used so that data are transmitted in a balanced mode.
Reliability is required of some of the signals handled in an automotive LAN because they may affect people's lives if they should be transmitted erroneously. SAE (Society of Automotive Engineers) has proposed a fault tolerant feature for a power short circuit in one of the aforementioned two buses, an open bus, and disconnection of one side of a terminating circuit connected to both ends of a bus.
As a system which provides such fault tolerant feature, there is, for instance, a multiplex transmission system according to Japanese Patent Application No. H2-113751 (published as Japanese Unexamined Patent Publication No. H4-10828 on Jan. 16, 1992). In this multiplex transmission system, three or more multiplex transmission lines are used and one line thereof is used as a control line. A master node connected to the multiplex transmission lines functions to detect a fault. If such fault is detected, then a controller IC or the like, which performs communication control, controls the voltage of the control line and secures the transmission of signals by the two buses other than the bus which has incurred the fault.
However, this multiplex transmission system presented a problem in that, if a failure such as software fault and hardware malfunction takes place in a controller IC, a controller circuit or the like, which controls the voltage of the control line, then the voltage of the control line becomes unstable. Particularly in the worst case, the voltage of the aforementioned control line almost "oscillates," preventing transmission in some cases.